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A diviner (aka divinist) is a spellcaster who specializes in the magic school of divination, which consists mostly of the accquisition of knowledge.
Divination is the school of magic dedicated to gathering information. Powerful divinations can allow the mage to see targets from a great distance, or even view what may normally be invisible. One of the most common uses of divination magic is scrying, which is the art of seeing something that may be far away - perhaps even on another plane of existence.
Diviners are, for example, found in the Scarlet Crusade.
See also: The Schools of Arcane Magic - Divination